Nassau County could owe millions of dollars in longevity pay to workers in its five major public unions after a state Supreme Court ruled that a disputed agreement made in the final months of the Mangano administration is valid.

State Supreme Court Justice Thomas Adams, in a Dec. 7 decision, found the September 2017 deal between former Chief Deputy County Executive Rob Walker and each of Nassau’s union leaders was “not the product of a mutual mistake of fact and is a valid and enforceable agreement between the parties.”

The agreement with Walker restored workers’ longevity pay, which has remained the same since 2011 when the county’s financial control board, NIFA, implemented a wage freeze.

Adams rejected the argument by county attorneys that Walker’s memorandum of agreement — which officials have said would have a total cost to the county of $10 million — was not legally binding.

It was not immediately clear whether the administration of County Executive Laura Curran, a Democrat, would appeal.

PBA president James McDermott said, “We won the ruling and I’m in talks with the county executive on how this will play out.”

The lawsuits, all filed separately, argue that the Walker agreement with the unions was “not enforceable because it is based upon a mutual mistake of fact.”

In a September 2017 letter, then-County Attorney Carnell Foskey said Walker’s agreements with the unions “exceed the ordinary terms and usual substance” of such deals, and would need approval from the county legislature and NIFA.

Adams, the county's administrative judge, ordered the county and employee unions to take up the matter in arbitration.

Curran took office Jan. 1, one day after expiration of the collective bargaining agreements with each of the five unions. County employees are working under the terms of the expired contracts. The unions represent about 7,000 employees.

Longevity pay rewards workers for their years of service. Union leaders say some workers are owed thousands of dollars.

Laricchiuta said the payments “are huge” for his members.

“I have members making $35,000 to $40,000 per year and this could mean $6,000 for them. It’s the difference between staying in your house or not,” he said.

Curran said previously she could not begin negotiations on new union contracts until the courts resolved the issue of longevity pay.

Walker, 43, a Republican and former state assemblyman from Hicksville, served as chief deputy to former County Executive Edward Mangano.

Walker was indicted in February on charges of obstructing justice and lying to FBI agents. He has pleaded not guilty, and his federal trial is set for Jan. 7.

The felony charges against Walker resulted from a federal investigation into whether Nassau public officials have taken money from county contractors and whether the parties tried to conceal their actions.